1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a CMOS image sensor capable of maximizing the focusing efficiency of light and achieving an improvement in performance thereof, and a method for manufacturing the same.
2. Discussion of the Related Art
Image sensors are semiconductor devices for converting an optical image into an electric signal. Image sensors are basically classified into charge coupled devices (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors.
A CMOS image sensor includes a matrix of photodiodes that sense light irradiated thereto, and a CMOS logic circuit unit to convert the sensed light into electric signals for data collection. The greater the light reception of the photodiodes, the better the photo-sensitivity of the image sensor.
In order to enhance the photo-sensitivity of the image sensor, there have been efforts to increase the ratio of an area of the photodiodes to the entire area of the image sensor, i.e. to increase a fill factor. In another conventional technology, the paths of light rays incident on regions other than the photodiodes are changed, so that the light rays are focused onto the photodiodes.
A representative example of the focusing technology as stated above is forming convex micro-lenses by use of a high light-transmitting material. The micro-lenses are located over the photodiodes to refract the paths of incident light rays, thereby allowing a greater amount of light to reach the photodiodes.
The micro-lenses refract light rays running parallel to an optical axis of the micro-lens to a predetermined focal point on the optical axis.
A prior art CMOS image sensor is explained with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating the configuration of a prior art CMOS image sensor.
As shown in FIG. 1, the prior art CMOS image sensor includes: one or more photodiodes 11 formed on a semiconductor substrate (not shown) to produce electric charges based on the amount of incident light; an inter-insulation layer 12 formed on the overall surface of the semiconductor substrate including the photodiodes 11; a protective layer 13 formed on the inter-insulation layer 12; red, green, and blue color filter layers 14 arranged on the protective layer 13 side by side, each to transmit light of a particular wavelength; a planarization layer 15 formed on the color filter layers 14; and one or more convex micro-lenses 16 having a predetermined curvature, the micro-lenses being formed on the planarization layer 15 for focusing light of a particular wavelength onto the photodiodes 11 by transmitting the light through a corresponding one of the color filter layers 14.
Although not shown in the accompanying drawings, the inter-insulation layer 12 contains an optical shielding layer to prevent light from reaching regions other than the photodiodes 11.
Instead of the photodiodes, as can be easily expected, photo gates may be used as devices for sensing light.
In the prior art, the curvature and height of the micro-lenses 16 are determined in consideration of various factors, such as a focal point of the focused light. Mainly, the micro-lenses 16 are made of a polymer based resin, and are fabricated via deposition, patterning, and reflow processes.
Specifically, the micro-lenses 16 must have an optimal size, thickness, and radius of curvature, which are determined by the size, position, and shape of each unit pixel, the thickness of the light sensing device, and the height, position, and size of the light shielding layer.
The height and radius of curvature of the micro-lenses 16 are determined in consideration of various factors, such as a focal point of the focused light, as stated above. A photosensitive resist layer is typically used to form the micro-lenses 16. After coating the photosensitive resist layer, the photosensitive resist layer is selectively patterned via photo-exposure and development processes, to form a photosensitive resist pattern. Subsequently, the photosensitive resist pattern is subjected to a reflow process.
The profile of the photosensitive resist pattern varies in accordance with a photo-exposure condition including a focal point.
For example, a process performing condition varies in accordance with forming requirements of thin sub-layers, resulting in a variation in the profile of the micro-lens.
Consequently, in a process for manufacturing the prior art CMOS image sensor having the above-described configuration, the micro-lenses 16 provided to enhance the focusing power of light are an important factor for determining the property of the image sensor.
If natural light is irradiated, the micro-lenses 16 serve to focus a greater amount of light onto the photodiodes 11 as light of a particular wavelength is transmitted through a corresponding one of the color filter layers 14.
Specifically, if light reaches the image sensor, the incident light is collected by the micro-lenses 16, and subsequently, is filtered while passing through the color filter layers 14, so that it is finally focused onto the photodiodes 11 arranged beneath the color filter layers 14 in a one-to-one ratio.
The light shielding layer serves to prevent the incident light from running along paths other than the predetermined paths.
The prior art method for manufacturing the CMOS image sensor as stated above, however, has a problem in that the patterning condition of the photosensitive resist pattern is very unstable. This degrades the focusing efficiency of light, resulting in a reduction in the operational performance of the CMOS image sensor.